Add Travis build

[gaborigloi]

First we could add an OPAM-based Travis build based on the docker-based Travis scripts in the ocaml-ci-scripts repo. We prefer the Docker-based .travis-docker.sh script in that repo to the normal .travis-opam.sh, because the former avoids building the OCaml 4.04.2 compiler each time, and therefore is a bit faster. The options supported by .travis-opam.sh are documented in this README, but .travis-docker.sh supports the same options, only the DISTRO and OCAML_VERSION environment variables differ.

To implement this, we can simply copy one of the existing Travis configurations, and check that it builds. Our repositories that use Travis are listed here: https://travis-ci.org/xapi-project The Travis config of xapi-nbd should be a good starting point for this repo: https://github.com/xapi-project/xapi-nbd/blob/master/.travis.yml

Let me know if you'd like to work on this, because first we need to enable Travis for this repo.